In order to format your Mac, you can use the OS X Utilities that are available on Mac OS X 10.6 and higher. To use the inbuilt utilities, you need to completely turn OFF your computer and, right after you hear the stat-up chime, press and hold the Command and R keys simultaneously. After a few seconds you will be shown the OS X Utilities options. From there, in order to format a hard disk on your computer, choose the Disk Utility.